Lowering Of Crystal Symmetry Research Articles

Extreme light confinement and control in low-symmetry phonon-polaritonic crystals

Extreme light confinement and control in low-symmetry phonon-polaritonic crystals

Ordinary light induced polarization zones in Tutton salt crystal

Ordinary light wave passing through a low symmetry crystal along an off optic -axis direction gets split into ordinary and extra-ordinary waves. If the beam size is large, the two polarized waves could overlap covering a large volume within the medium. It is realized that overlapping ordinary and extra-ordinary waves, result in creation of alternate polarization regions with different fluctuating crystalline fields inside a low - symmetry hydrated crystal. If suitable ions are doped in an experimental crystal, their optical absorption spectra could reflect the effect of alternate polarization fields and creation of ordinary light induced periodic polarization (OLIPP) effect inside a low symmetry hydrated crystal.

Weighted Burgers Vector analysis of orientation fields from high-angular resolution electron backscatter diffraction

The Weighted Burgers Vector (WBV) method can extract information about dislocation types and densities present in distorted crystalline materials from electron backscatter diffraction (EBSD) maps, using no assumptions about which slip systems might be present. Furthermore, high-angular resolution EBSD (HR-EBSD) uses a cross-correlation procedure to increase the angular precision of EBSD measurements by an order of magnitude compared to conventional EBSD. However, the WBV technique has not previously been applied to HR-EBSD data and therefore it remains unclear as to which low-angle substructures can be reliably characterised by WBV analysis of conventional EBSD data and which require additional HR-EBSD processing. To establish some practical examples that can be used to guide future data-acquisition strategies, we compare the output of the WBV method when applied to conventional EBSD data and HR-EBSD data collected from the most common minerals in Earth's lower crust (plagioclase feldspar) and upper mantle (olivine). The results demonstrate that HR-EBSD and WBV processing are complementary techniques. The increase in angular precision achieved with HR-EBSD processing allows low-angle (on the order of 0.1°) structures, which are obscured by noise in conventional EBSD data, to be analyzed quantitatively using the WBV method. Combining the WBV and HR-EBSD methods increases the precision of calculated WBV directions, which is essential when using information about active slip systems to infer likely deformation mechanisms from naturally deformed microstructures. This increase in precision is particularly important for low-symmetry crystals, such as plagioclase, that have a wide range of available slip systems that vary in relative activity with changing pressure, temperature and differential stress. Because WBV directions are calculated using no assumptions about which slip systems may be present, combining this technique with HR-EBSD to refine the precision of lattice orientation gradients is ideal for investigating complex natural materials with unknown deformation histories.

Polarization-tuneable excitonic spectral features in the optoelectronic response of atomically thin ReS2

The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence of dichroic optical and optoelectronic response, absent in other layered transition metal dichalcogenides, which could be exploited for device applications requiring polarization resolution. To date, spectroscopy studies on the optical response of ReS2 have relied almost exclusively in characterization techniques involving optical detection, such as photoluminescence, absorbance, or reflectance spectroscopy. However, to realize the full potential of this material, it is necessary to develop knowledge on its optoelectronic response with spectral resolution. In this work, we study the polarization-dependent photocurrent spectra of few-layer ReS2 photodetectors, both in room conditions and at cryogenic temperature. Our spectral measurements reveal two main exciton lines at energies matching those reported for optical spectroscopy measurements, as well as their excited states. Moreover, we also observe an additional exciton-like spectral feature with a photoresponse intensity comparable to the two main exciton lines. We attribute this feature, not observed in earlier photoluminescence measurements, to a non-radiative exciton transition. The intensities of the three main exciton features, as well as their excited states, modulate with linear polarization of light, each one acquiring maximal strength at a different polarization angle. We have performed first-principles exciton calculations employing the Bethe-Salpeter formalism, which corroborate our experimental findings. Our results bring new perspectives for the development of ReS2-based nanodevices.

Room temperature field-free switching of perpendicular magnetization through spin-orbit torque originating from low-symmetry type II Weyl semimetal

Spin-orbit torque (SOT) is a promising strategy to deterministically switch the perpendicular magnetization, but usually requires an in-plane magnetic field for breaking the mirror symmetry, which is not suitable for most advanced industrial applications. Van der Waals (vdW) materials with low crystalline symmetry and topological band structures, e.g., Weyl semimetals (WSMs), potentially serve as an outstanding system that may simultaneously realize field-free switching and high energy efficiency. Yet, the demonstration of these superiorities at room temperature has not been realized. Here, we achieve a field-free switching of perpendicular magnetization by using a layered type II WSM, TaIrTe 4 , in a TaIrTe 4 /Ti/CoFeB system at room temperature with the critical switching current density ~2.4 × 10 6 A cm −2 . The field-free switching is ascribed to the out-of-plane SOT allowed by the low crystal symmetry. Our work suggests that using low-symmetry materials to generate SOT is a promising route for the manipulation of perpendicular magnetization at room temperature.

Restoration of the orientation distribution function for materials with low lattice and sample symmetry using the harmonic method

A lot of the properties polycrystalline materials depend on their crystallographic texture. The most complete information about the texture can be obtained from the orientation distribution function (ODF). We present the results of recovering ODF using series expansion technique for materials with low crystal and sample symmetry. The technique of ODF restoration is based on its Fourier series expansion with symmetrical spherical harmonic functions. Real spherical harmonics which are linear combinations of general spherical harmonics were used. The model single-component texture as well as the real texture of magnesium alloy sample subjected to equal-channel angular pressing have been studied. Textures are characterized by hexagonal crystal symmetry and triclinic sample symmetry. In both cases RP-factors and ODF calculation errors that were used as reliability criteria of ODF reconstruction showed good agreement between the calculated and experimental data. It was also revealed that the ODF of a magnesium alloy sample subjected to equal-channel angular pressing contains two texture components (1216)[1211] and (1216)[1211] with maximum intensity values of 13.81 and 2.23, respectively. The results obtained can be used for texture studies of ceramics, rocks and other non-metallic materials characterized by a lower symmetry.

P3‐Na0.45Ni0.2Mn0.8O2/Na2SeO4 Heterostructure Enabling Long‐Life and High‐Rate Sodium‐Ion Batteries

AbstractSodium‐based layered oxide cathodes are competitive candidates for commercial sodium‐ion batteries owing to their high theoretical capacities, low costs, and simple synthesis. P3‐type layered oxides with large open channels enable fast Na+ transport and hence good rate performance. However, the lower crystal symmetry of P3‐type oxides and variation of Na+ contents in the Na layer during desodiation/sodiation lead to large electrostatic repulsion changes between TMO2 slabs (TM=Transition Metal), resulting in irreversible phase transitions, and fast performance degradation. Herein, a potential Na+ conductor Na2SeO4 is first found that it can be easily in situ grown on P3‐Na0.45Ni0.2Mn0.8O2 to form a novel heterostructure P3‐Na0.45Ni0.2Mn0.8O2/Na2SeO4. The synergy between P3‐Na0.45Ni0.2Mn0.8O2 and Na2SeO4 functions in promoting Na+ diffusion and suppressing P3‐O3 phase transitions upon deep sodiation, which results in recorded high‐rate capability (68.2% capacity retention with retained 83.9 mAh g−1 capacity at 6400 mA g−1) and superior cycling stability (capacity retention 75% after 1000 cycles) among all reported P3‐type cathodes. Thus, it is believed that this novel heterostructure design opens a new pathway to promote practical applications for layered oxide cathodes in sodium‐ion batteries.

Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide

Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.

Atomic scale mechanism of β to γ phase transformation in gallium oxide

We report the detailed mechanism behind the β to γ phase transformation in Sn-doped and Si-implanted Ga2O3 that we determined based on the direct observation of the atomic scale structure using scanning transmission electron microscopy (STEM). Quantitative analysis of the STEM images revealed that the high concentration of impurity atoms favored the formation of interstitial–divacancy complexes, which then leads to the secondary relaxation that creates additional interstitial atoms and cation vacancies, resulting in a local structure that closely resembles γ-Ga2O3. We explain the mechanism of how the impurity atoms facilitate the transformation, as well as the detailed sequence of the local γ phase transformation. The findings here offer an insight on how the lattice respond to the external stimuli, such as doping and strain, and transform into different structures, which is important for advancing Ga2O3 but also a variety of low symmetry crystals and oxides with multiple polymorphs.

Band Structure Evolution during Reversible Interconversion between Dirac and Standard Fermions in Organic Charge-Transfer Salts

Materials containing Dirac fermions (DFs) have been actively researched because they often alter electrical and magnetic properties in an unprecedented manner. Although many studies have suggested the transformation between standard fermions (SFs) and DFs, the non-availability of appropriate samples has prevented the observation of the transformation process. We observed the interconversion process of DFs and SFs using organic charge-transfer (CT) salts. The samples are unique in that the constituents (the donor D and acceptor A species) are particularly close to each other in energy, leading to the temperature- and D-A-combination-sensitive CT interactions in the solid states. The three-dimensional weak D–A CT interactions in low-symmetry crystals induced the continuous reshaping of flat-bottomed bands into Dirac cones with decreasing temperature; this is a characteristic shape of bands that converts the behavior of SFs into that of DFs. Based on the first-principles band structures supported by the observed electronic properties, round-apex-Dirac-cone-like features appear and disappear with temperature variation. These band-structure snapshots are expected to add further detailed understanding to the related research fields.

Source-configured symmetry-broken hyperbolic polaritons

Polaritons are quasi-particles that combine light with matter, enabling precise control of light at deep subwavelength scales. The excitation and propagation of polaritons are closely linked to the structural symmetries of the host materials, resulting in symmetrical polariton propagation in high-symmetry materials. However, in low-symmetry crystals, symmetry-broken polaritons exist, exhibiting enhanced directionality of polariton propagation for nanoscale light manipulation and steering. Here, we theoretically propose and experimentally demonstrate the existence of symmetry-broken polaritons, with hyperbolic dispersion, in a high-symmetry crystal. We show that an optical disk-antenna positioned on the crystal surface can act as an in-plane polarized excitation source, enabling dynamic tailoring of the asymmetry of hyperbolic polariton propagation in the high-symmetry crystal over a broad frequency range. Additionally, we provide an intuitive analysis model that predicts the condition under which the asymmetric polaritonic behavior is maximized, which is corroborated by our simulations and experiments. Our results demonstrate that the directionality of polariton propagation can be conveniently configured, independent of the structure symmetry of crystals, providing a tuning knob for the polaritonic response and in-plane anisotropy in nanophotonic applications.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications.

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons.

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Polymorphism in LiDy(WO4)2: Phase tunable synthesis, neutron diffraction, and symmetry-driven upconversion red emission

Tunable polymorphic structures to achieve novel properties are of great concern for energy-related applications. Herein, we demonstrate temperature-driven irreversible structural phase transition in LiDy(WO4)2 (β-LiDyW and α-LiDyW). A facile sol-gel method has been employed to achieve phase pure crystalline polymorphs at relatively lower temperatures and time. LiDy(WO4)2 crystallizes in a monoclinic wolframite-type structure (space group, P21/n, No = 14) at ambient temperature and a Scheelite-like tetragonal structure (space group, I41/a, No = 88) upon heating at high temperature. Crystal structure analysis shows that Li+ and Dy3+ occupy the distinct site, and W forms a distorted WO6 octahedron in the β-LiDyW phase. In contrast, Li+ and Dy3+ are statistically distributed on a dodecahedral S4 site sharing a similar crystallographic lattice site, and W forms a WO4 tetrahedron in the α-LiDyW phase. The Wolframite to Scheelite (β → α) transformation is due to the increase in the crystal symmetry on the heating function where distorted WO6 transformed to free WO4 going from β → α phase and coordination of W+6 is lowered. Density functional theory calculations at 0 K revealed that the β-LiDyW is energetically more favorable than α-LiDyW by 337.3 meV per formula unit. The co-substitution of the Yb3+−Er3+ pair in LiDy(WO4)2 lattice displays a concentration-dependent upconversion red emission when excited with 980 nm. The monoclinic phase of LiDy0.7Yb0.2Er0.1(WO4)2 shows an intense red emission at 656 nm due to inherent lower crystal symmetry. Distorted WO6 and chemically induced lattice distortions in β-LiDy0.7Yb0.2Er0.1(WO4)2 would have strongly influenced the coordination environment of the Er3+ to achieve red emission. This study presents the structural relationship among the tunable crystallographic phases of LiDy(WO4)2 with the observed red emission induced by Yb3+−Er3 upon 980 nm irradiation.

Quasi-One-Dimensional ZrS3 Nanoflakes for Broadband and Polarized Photodetection with High Tuning Flexibility.

Two-dimensional (2D) layered materials with low crystal symmetries have exhibited unique anisotropic physical properties. Here, we report systematic studies on the photoresponse of field effect transistors (FETs) fabricated using quasi-one-dimensional ZrS3 nanoflakes. The as-fabricated phototransistors exhibit a broadband photocurrent response from ultraviolet to visible regions, where the responsivity and detectivity can be enhanced via additional gate voltages. Furthermore, benefiting from the strong in-plane anisotropy of ZrS3, we observe a gate-voltage and illumination wavelength-dependent polarized photocurrent response, while its sub-millisecond-time response speed is also polarization-dependent. Our results demonstrate the flexible tunability of photodetectors based on anisotropic layered semiconductors, which substantially broadens the application of low symmetry layered materials in polarization-sensitive optoelectronic devices.

Spin-orbit torque for field-free switching in C3v crystals

Spin-orbit torques in noncentrosymmetric polycrystalline magnetic heterostructures are usually described in terms of field-like and damping-like torques. However, materials with a lower symmetry point group can exhibit torques whose behavior substantially deviates from the conventional ones. In particular, based on symmetry arguments it was recently proposed that systems belonging to the ${\text{C}}_{3\mathrm{v}}$ point group display spin-orbit torques that can promote field-free switching [Liu et al., Nat. Nanotechnol. 16, 277 (2021)]. In the present work, we analyze the general form of the torques expected in ${\text{C}}_{3\mathrm{v}}$ crystals using the invariant theory. We uncover several new components that arise from the coexistence of the threefold rotation and mirror symmetries. Using both tight binding model and first principles simulations, we show that these unconventional torque components arise from the onset of trigonal warping of the Fermi surface and can be as large as the damping-like torque. In other words, the Fermi surface warping is a key indicator to the onset of field-free switching in low symmetry crystals.

Accurate determination of elastic anisotropy to detect internal microstructures using resonant ultrasound spectroscopy

Elastic constants are of great significance, because they are fundamental thermodynamic susceptibilities that connect directly to thermodynamics and electronic structure, as well as to mechanical properties. Resonant ultrasound spectroscopy (RUS) measures non-destructively all fundamental elastic properties by determining the natural frequencies at which a 3D object resonates when is mechanically excited (normal modes). By using an inversion scheme, it is possible to extract the entire elastic tensor of a material with extreme sensitivity in a single frequency sweep using only one, small sample. This makes RUS particularly advantageous to study physical properties of low-symmetry single crystals and textured bulk materials. I will present recent RUS measurement examples of its applications to thermodynamic and materials science problems. I will focus on the effect of texture in elastic constants as a result of mechanical or growth processes and the robustness of results obtained by RUS. In materials such as aluminum that is often considered isotropic but shows clear anisotropy when it is extruded, or 3D printed. Similar but much larger effect is found in additive manufactured stainless steel where the growth process leads to a large anisotropy with a softening of 40%–50% along the build direction.

Structures, Thermal Properties, and Reactivities of Cationic Rh-cod Complexes in Solid State (cod = 1,5-Cyclooctadiene).

Cationic rhodium complexes with 1,5-cyclooctadiene (cod) ligands are important organometallic compounds that are useful as precatalysts; however, their solid-state structures and thermal properties have not been adequately investigated. In this study, we synthesized [Rh(cod)L]X (L = cod, C6H6, PhMe; X = SbF6, (FSO2)2N (= FSA), CF3BF3, CB11H12) and investigated their phase behaviors, crystal structures, and reactivities. The phase transitions of these salts result in disordered solid-state structures. Moreover, the structural disorder increases with a decrease in the cation symmetry in the SbF6 salts; [Rh(cod)(PhMe)]SbF6 exhibits a rotator phase, and the cations in other salts exhibit a dynamic rotational disorder. In contrast, a lower crystal symmetry with less cation disorder is observed for FSA salts. The thermal stabilities and reactivities of these salts were further investigated. FSA salts with arene ligands produce anion-coordinated complexes upon melting, and SbF6 salts with arene ligands produce [Rh(cod)L'2]SbF6 (L' = MeCN and SMe2) via an in situ single-crystal-to-single-crystal ligand-exchange reaction.

Hidden interplay of current-induced spin and orbital torques in bulk Fe3GeTe2

Low crystal symmetry of magnetic van der Waals materials naturally promotes spin-orbital complexity unachievable in common magnetic materials used for spin-orbit torque switching. Here, using first-principles methods, we demonstrate that an interplay of spin and orbital degrees of freedom has a profound impact on spin-orbit torques in a prototype van der Waals ferromagnet: Fe$_3$GeTe$_2$ (FGT). While we show that bulk FGT hosts strong "hidden" current-induced torques harvested by each of its layers, we uncover that their origin alternates between the conventional spin flux torque and the so-called orbital torque as the magnetization direction is varied. A drastic difference in the behavior of the two types of torques results in a non-trivial evolution of switching properties with doping. Our findings promote the design of non-equilibrium orbital properties as the guiding mechanism for crafting the properties of spin-orbit torques in layered van der Waals materials.

Oriented CuWO4 Films for Improved Photoelectrochemical Water Splitting.

Hydrogen generation through photoelectrochemical (PEC) technology is one of the most appropriate ways for delivering sustainable fuel. Simultaneously, anisotropic properties will be exhibited by the materials with low crystal symmetry, allowing the tuning of the PEC properties by controlling the crystallographic orientation and exposed facets. Therefore, we synthesized copper tungstate films (CuWO4) with highly exposed (100) crystal facets by regulating anions in the precursor solution. According to experimental characterization and density functional theory calculations, the CuWO4 film with a high exposure ratio of the (100) crystal facet has promoted charge transport with trap-free mode and reduced recombination of electrons and holes. Meanwhile, the oxygen evolution reaction is promoted on the (100) facet because of the relatively low energy barrier. Compared to the CuWO4 with other mixed exposure facets, CuWO4 with a highly exposed (100) facet presents a twofold current density (0.38 mA/cm2) and one-fifteenth electron transit time (0.698 ms) and also has great stability (more than 6 h). These results provide an easy way to enhance the PEC performance by modulating the exposure facets of the film electrode.